Relative paths in a <repo>.git/objects/info/alternates file are to
be expanded relative to the <repo>.git/objects directory.
Previously, they were expanded relative to the <repo>.git directory.

The current code checks in both locations for backward compatibility
with grit <= 2.3.0 but I'd rather rip it out completely.

allow Index#commit to specify separate authors and committers with correct timezones for the timestamps

This speeds things up considerably when output is large. Especially
so under some versions of Rails (AS) where String#+ is made to be
extremely slow.

With 500K blob, before:

    $ ruby bench-string-concat.rb 12044a76034e894c2412aea9c20508b5c8277784
    blob: 12044a76034e894c2412aea9c20508b5c8277784
    size: 499855
                                        user     system      total        real
    native cat_file                11.030000   9.140000  20.230000 ( 21.182163)
    ruby cat_file                   0.040000   0.020000   0.060000 (  0.061033)

ActiveSupport makes it even worse:

    $ ruby -ractive_support bench-string-concat.rb 12044a76034e894c2412aea9c20508b5c8277784
    blob: 12044a76034e894c2412aea9c20508b5c8277784
    size: 499855
                                        user     system      total        real
    native cat_file                15.830000   9.570000  25.470000 ( 26.286903)
    ruby cat_file                   0.050000   0.020000   0.070000 (  0.073557)

With this commit applied (using String#<< instead of String#+):

    $ ruby bench-string-concat.rb 12044a76034e894c2412aea9c20508b5c8277784
    blob: 12044a76034e894c2412aea9c20508b5c8277784
    size: 499855
                                        user     system      total        real
    native cat_file                 0.310000   0.440000   0.800000 (  1.765703)
    ruby cat_file                   0.040000   0.020000   0.060000 (  0.064426)

The bench-string-concat.rb script is here:

<https://gist.github.com/e749138b066ed9d81fbe>

explicitly make the Grit data classes lazily loaded (instead of every single ruby object).  Also, add attr_writer(:lazy_source) for lazy objects

* Add current directory to path for tests because 1.9 does not
* Explicity open file in binary mode to get around encoding issues